U.S. patent number 7,758,671 [Application Number 11/503,712] was granted by the patent office on 2010-07-20 for versatile dehumidification process and apparatus.
This patent grant is currently assigned to Nanocap Technologies, LLC. Invention is credited to Jack N. Blechner, Arthur S. Kesten.
United States Patent |
7,758,671 |
Kesten , et al. |
July 20, 2010 |
Versatile dehumidification process and apparatus
Abstract
A process and apparatus for dehumidifying a gas stream is
provided. The apparatus includes a single semi-permeable osmotic
membrane, at least one gas stream compartment formed in part by the
osmotic membrane, and at least one osmotic fluid compartment formed
in part by the osmotic membrane. The semi-permeable osmotic
membrane has randomly arranged pores disposed across a thickness
extending between a first side and a second side, and wherein some
of the pores are small enough to permit capillary condensation
within the membrane, leading to condensate travel across the
thickness of the single membrane without requiring a separate
capillary condenser, and which single membrane restricts transport
of the osmotic fluid across the thickness of the membrane. The
first side of the osmotic membrane is exposed to the gas stream
compartment, and the second side of the osmotic membrane is exposed
to the osmotic fluid compartment.
Inventors: |
Kesten; Arthur S. (West
Hartford, CT), Blechner; Jack N. (West Hartford, CT) |
Assignee: |
Nanocap Technologies, LLC (West
Hartford, CT)
|
Family
ID: |
38666844 |
Appl.
No.: |
11/503,712 |
Filed: |
August 14, 2006 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20080034966 A1 |
Feb 14, 2008 |
|
Current U.S.
Class: |
95/46; 95/43;
96/10; 96/4; 95/45; 95/52 |
Current CPC
Class: |
B01D
53/229 (20130101); F24F 3/147 (20130101); F24F
3/1417 (20130101); B01D 53/268 (20130101); F24F
2003/1435 (20130101); F24F 2003/144 (20130101) |
Current International
Class: |
B01D
53/22 (20060101); B01D 59/12 (20060101) |
Field of
Search: |
;95/43,45,52,46
;96/4,10 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
The American Heritage Dictionary of the English Language: Fourth
Edition 2000. cited by examiner .
Eijkel et al. (Water in micro- and nanofluidics systems described
using the water potential) This journal is The Royal Society of
Chemistry, Sep. 29, 2005 Lab Chip, 2005, 5, 1202-1209. cited by
examiner .
Yan et al. (Proton exchange membrane with hydrophilic capillaries
for elevated temperature PEM fuel cells) Electrochemistry
Communications 11 (2009) 71-74. cited by examiner .
Eijkel et al. (Osmosis and pervaporation in polyimide submicron
microfluidic channel structures) Applied Physics Letters 87, 114103
2005. cited by examiner.
|
Primary Examiner: Greene; Jason M
Assistant Examiner: Shumate; Anthony
Attorney, Agent or Firm: O'Shea Getz P.C.
Claims
What is claimed is:
1. An apparatus for dehumidification of a gas stream, comprising: a
single semi-permeable hydrophilic osmotic membrane, which membrane
comprises randomly arranged pores disposed across a thickness
extending between a first side and a second side, and wherein some
of the pores are small enough to permit capillary condensation
within the membrane, leading to condensate travel across the
thickness of the single membrane without requiring a separate
capillary condenser, and which single membrane restricts transport
of an osmotic fluid across the thickness of the membrane; at least
one gas stream compartment through which the gas stream may flow,
formed in part by the osmotic membrane, wherein the first side of
the osmotic membrane is positioned so as to be exposed to the gas
stream within the gas stream compartment; at least one osmotic
fluid compartment formed in part by the osmotic membrane, wherein
the second side of the osmotic membrane is contiguous with the
osmotic fluid compartment; and an osmotic fluid regenerator.
2. A process for dehumidifying a gas stream, comprising the steps
of: providing an osmotic fluid; providing a single semi-permeable
hydrophilic osmotic membrane, which membrane comprises randomly
arranged pores disposed across a thickness extending between a
first side and a second side, and wherein some of the pores are
small enough to permit capillary condensation within the membrane,
leading to condensate travel across the thickness of the single
membrane without requiring a separate capillary condenser, and
which single membrane restricts transport of the osmotic fluid
across the thickness of the membrane; placing the osmotic fluid in
a compartment formed in part by the semi-permeable membrane,
wherein the second side of the osmotic membrane is exposed to the
osmotic fluid; exposing the first side of the osmotic membrane to
the gas stream to be dehumidified; maintaining a sufficiently high
water concentration gradient across the osmotic membrane during the
dehumidification process to result in a flux of water through the
osmotic membrane; and regenerating the osmotic fluid to maintain a
high concentration of solute in the osmotic fluid.
3. The process of claim 2, wherein the step of regenerating the
osmotic fluid includes the step of evaporating excess water from
the osmotic fluid.
4. The process of claim 2, wherein the step of regenerating the
osmotic fluid includes the step of adding solute to the osmotic
fluid.
Description
TECHNICAL FIELD
This invention relates to processes and apparatus for conditioning
a gas stream, such as air, and to the dehumidification of a gas
stream in particular.
BACKGROUND ART
Conditioning of a gas stream, such as air, generally involves the
removal or addition of moisture and the increase or decrease of
temperature to make the gas stream suitable for its intended
environment. For air conditioning in warm weather, this typically
involves dehumidifying and cooling the air to comfortable
levels.
Current dehumidification technology is commonly based on the
conventional, refrigerant vapor compression cycle (hereinafter
referred to as DX technology) or on desiccant substrate capture
technology (hereinafter referred to as DS technology). DX
technology requires cooling humid supply air, such as the air
within a room and/or outside air, to the water vapor condensation
point, with external heat rejection on the compression side. This
usually requires the supply air to be cooled below comfortable
temperatures and, thereafter, either reheated or mixed with warmer
air to raise its temperature to an acceptable level before
directing it into the space being dehumidified. Twenty to
thirty-five percent (20-35%) of the energy expended in cooling the
high humidity air is utilized to remove the latent heat from the
air (the heat of condensation associated with water vapor
condensation). Cooling and dehumidification of the air are thus
coupled. That makes it impossible to independently control comfort
parameters, making the DX cycle less efficient, from an overall
system perspective, than a technology that would allow independent
control of sensible and latent heat.
In applications where the outside air has both high humidity and
temperature and the functional use of the interior space generates
high water vapor levels (e.g., populated convention halls, exercise
rooms, school buildings, etc.), it may not be possible for the DX
technology to maintain the air introduced into the interior space
at the correct humidity and temperature for maintaining comfort.
The air delivered is cool but "muggy", since further cooling to
remove additional water would result in the air being uncomfortably
cool.
In stand-alone dehumidification using a conventional compression
cycle, heat reject is in direct contact with the room air. As a
consequence, the room air becomes more comfortable from a humidity
side, but may be less comfortable (too warm) from a temperature
parameter consideration. Again the comfort parameters are
coupled.
DS systems are generally applied in central air, ducted systems.
Water vapor is captured by capillary condensation on a solid phase
substrate containing pores of the appropriate size (typically less
than 100 Angstroms) to cause capillary condensation. The capture
process is efficient and rapid. However, removal of the water vapor
from the pores, wherein the intrinsic vapor pressure of the water
is lowered in correspondence with the Kelvin equation, requires
energy input. It also requires removing the substrate from the high
humidity air stream and placing it in an exhaust, water reject
stream, before adding the re-evaporation energy. Alternatively, the
substrate may remain fixed and the treated air and exhaust streams
flow directions interchanged as is done in a parallel bed,
desiccant drier system.
In these DS systems, the re-evaporation energy is the latent heat
of condensation plus the heat of adsorption of the water vapor in
the substrate pore material. It is important to note that DS
technology requires, in steady state operation, the addition of
this energy at a rate equal to or greater than the latent heat of
condensation of water in the desiccant substrate. That is, the
water vapor reject power input must exceed the equivalent latent
heat of condensation power. After water removal from the desiccant
substrate, the substrate must be re-cooled to the water capture
temperature range of the substrate. As a consequence, some of the
sensible heat of the subsequent cooling system (e.g., a DX cooling
system) must be utilized in treating the DS substrate rather than
for cooling the now dehumidified air.
The advantage of DS technology is that humidity levels in the
outside air and/or recirculated air can be adjusted independently
of the subsequent cooling step. The disadvantage is the requirement
to move the substrate and treated air stream relative to each other
for capture and rejection of the water vapor. This requires moving
a large substrate through a sealed system, or, in a parallel bed DS
system, requires complicated valving and valve cycling to move the
humid air stream and an exhaust stream alternately across the
desiccant beds. Again, application in typical stand alone,
non-ducted room-type dehumidifiers is difficult if not
impossible.
U.S. Pat. No. 6,539,731 discloses another type of dehumidification
technology that utilizes a porous wall separating humid air from an
osmotic fluid. The porous wall includes a capillary condenser layer
and an osmotic layer. The capillary condenser layer is formed from
ceramic materials having pores sized small enough to permit water
vapor within the air to condense into liquid form. An osmotic
driving force, resulting from a water concentration gradient,
transports the condensed water through the capillary condenser
layer and the contiguous osmotic layer, and into the osmotic fluid.
The osmotic layer prevents substantially all of the osmotic fluid
from entering the capillary condenser. This type of device is
effective in promoting water transport at fluxes in excess of 1
liter/square meter-hour. Under some circumstances, however, the
pore structure of the capillary condenser can become unstable and
degrade over a relatively short period of time in a humid
environment. In addition, the capillary condenser is typically made
from a rigid material and is therefore limited to those
applications where a rigid body is acceptable; i.e., the rigid
capillary condenser cannot be used in those applications where a
flexible device is required.
What is needed is a process and an apparatus for dehumidifying a
gas stream that overcomes the deficiencies of the prior art.
DISCLOSURE OF INVENTION
An object of the present invention is to provide an efficient
method and means for removing water from an gas stream wherein the
level of dehumidification is not interdependent with the
temperature to which that gas stream may need to be ultimately
cooled (for comfort or other purposes) before it is exhausted into
the space being conditioned.
According to an aspect of the present invention, a process for
dehumidifying a gas stream is provided that includes steps of: a)
providing a semi-permeable wall having an osmotic membrane with a
plurality of pores at least some of which are operably sized to
permit capillary condensation, a first side, and a second side; b)
placing an osmotic fluid in a compartment formed in part by the
semi-permeable wall, wherein the second side of the osmotic
membrane is exposed to the osmotic fluid; c) exposing the first
side of the osmotic membrane to the gas stream to be dehumidified;
and d) maintaining a sufficiently high water concentration gradient
across the osmotic membrane during the dehumidification process to
result in a flux of water through the osmotic membrane in the
direction from the first side toward the second side.
According to another aspect of the present invention, an apparatus
for dehumidification of a gas stream is provided. The apparatus
includes at least one semi-permeable osmotic wall, at least one gas
stream compartment formed in part by the osmotic wall, and at least
one osmotic fluid compartment formed in part by the osmotic wall.
Each semi-permeable osmotic wall has an osmotic membrane with some
pores small enough to permit capillary condensation, a first side,
and a second side. The first side of each osmotic membrane is
exposed to the gas stream compartment, and the second side of each
osmotic membrane is exposed to the osmotic fluid compartment.
In both the present invention process and apparatus, the osmotic
fluid compartment is formed in part by the second side of the
semi-permeable wall. In most instances, the gas stream compartment
is formed in part by the first side of the semi-permeable wall. The
gas stream compartment may not be a complete enclosure, and may,
for example, assume the form of ducting through which the gas
stream may be passed. The gas stream (e.g., high humidity outdoor
air and/or recirculated indoor air) is brought into and through the
gas stream compartment and passes over the first side of the
osmotic membrane. Water vapor in the gas stream condenses and
travels through the membrane in the direction from the first side
toward the second side and into the osmotic fluid. When the water
travels through the membrane, it will likely not travel exclusively
in a direction perpendicular to both the first and second sides.
Rather, the path through the osmotic membrane will likely include a
lateral component as well as a perpendicular component. Overall,
however, the path taken by the water through the membrane may be
described as traveling in the direction from the first side to the
second side. The now less humid gas stream exits the gas stream
compartment and may then be cooled by separate air conditioning
apparatus, if desired.
The semi-permeable osmotic wall typically includes a macroporous
support with the osmotic membrane. The thickness of a typical
semi-permeable osmotic membrane is less than 100 nanometers. The
membrane comprises a plurality of pores randomly arranged across
its thickness, formed by internal surface configurations within the
osmotic membrane. Some of the pores are formed by surfaces
positioned close enough to one another to permit capillary
condensation. Our investigations lead us to believe that liquid
formed within these pores connects with liquid formed in adjacent
pores, collectively forming continuous paths of liquid (referred to
herein as "liquid bridges"). These "liquid bridges", formed as we
believe or otherwise, extend across the thickness of the
semi-permeable membrane and thereby provide paths by which water
can travel collectively in the direction from the first side toward
the second side of the osmotic membrane. Because the membrane is so
thin, water concentration gradients across the membrane can be
large. This can provide a large driving force for water transport
between the humid air and osmotic fluid.
In some embodiments the osmotic fluid is a solute dissolved in
water, wherein the solute has a high ion (e.g., a salt)
concentration, and the osmotic layer is a membrane permeable to
water, but not to ions in solution. The choice of solute and any
other additives making up the osmotic fluid will be determined by
the transport properties through the membrane. For example, the
solute and the osmotic membrane are selected such that the size of
the hydrated solute molecules is greater than the pore size of the
osmotic membrane in order to prevent the solute from flowing
through the osmotic membrane toward the humid air. The solute is
also selected to ensure that the molecules of solute do not lodge
within and thereby block the pores of the semi-permeable osmotic
membrane. To assure that the condensed water flows from the
semi-permeable membrane into the osmotic fluid, a high
concentration of solute is maintained in the osmotic fluid to
maintain a high water concentration gradient across the
membrane.
In some embodiments, the osmotic fluid is one that is miscible with
water at all concentrations, such as glycerol or ethylene glycol.
Here the fluid can be maintained at a low water concentration in
order to maximize the osmotic flux. Typical membranes have
permeabilities for glycerol which are about one thousand times less
than for water. However, some reverse transport may occur.
In some cases, a biocidal component may be added to the osmotic
fluid in conjunction with a solute chosen for maximum flux through
the membrane. The biocidal component is selected to prevent
microbial growth or biofouling on surfaces which would naturally
occur in an aqueous environment and eventually block the membrane
or pores. Examples of biocidal or bacteristatic additives that can
exist in osmotic fluid as ionic species include silver and copper.
In addition to these simple ionic antimicrobial agents, small
concentration of larger molecules such as quaternary amines, or
gluteraldehydes may be used. Gluteraldehyde is an example of a
sterilant and disinfectant that is less corrosive than most other
chemicals and does not damage plastics. Bleach (e.g., hypochlorous
acid), for example, is antimicrobial but accelerates corrosion and
would not be a preferred additive to the osmotic fluid.
The high solute concentration of the osmotic fluid may be
maintained in several different ways, including: 1) excess water
may be evaporated or otherwise removed from the fluid; 2) the
solute may be replenished at appropriate times or intervals; and/or
3) the fluid may be provided with excess solute (undissolved) that
dissolves automatically when the concentration of water in the
osmotic fluid exceeds the amount needed to have the water fully
saturated by the solute. Other techniques or a combination of
techniques may also be used to maintain a high solute
concentration.
The dehumidification apparatus preferably, however, includes means
for regenerating the osmotic fluid to maintain a high concentration
of solute in the osmotic fluid, and thus to maintain the high water
concentration gradient across the osmotic layer during operation of
the apparatus. The regenerating means may, for example, include
apparatus operable to evaporate, either continuously or as needed,
excess water from the osmotic fluid.
One of the primary benefits of the present invention is that the
humidity of the incoming air may be controlled independently of the
temperature. The water may be condensed out of the incoming humid
gas stream, taking advantage of the ability of the osmotic wall to
rapidly and efficiently remove water vapor without the need to
remove sensible heat from the air stream (i.e., the moisture may be
removed from the gas stream at ambient temperatures). The water
condensed in the osmotic membrane is caused to move into the
osmotic fluid by maintaining a water concentration gradient across
the membrane. The water concentration gradient across the osmotic
membrane is created and maintained by having a sufficiently low
concentration of water (i.e., a high concentration of solute or
miscible fluid) within the osmotic fluid. If the water vapor
removed from the air is rejected to an exhaust area not in contact
with the treated air, the now dehumidified gas stream may then be
cooled to any desired temperature by appropriate means, such as by
using a standard air conditioning cycle. The incoming air stream is
thus made more comfortable by separately controlling both its
humidity and temperature.
Another advantage provided by the present invention is that a gas
stream can be dehumidified using less energy than is required using
prior art methods. For example, re-evaporation power requirements
for the present invention are lower than if the water were to be
removed from the system by, for example, reheating a desiccant bed.
This is because the osmotic fluid serves as a latent energy buffer
for the captured water vapor (i.e., the heat of condensation
released when water vapor condenses is buffered by the osmotic
fluid). While it may be necessary or desirable to use an energy
source to assist in the removal (e.g., by separation or
re-evaporation) of the excess water from the osmotic fluid, the
process can be relatively simple and energy efficient compared, for
example, to the analogous step of a DS cycle wherein a bed of
desiccant is usually taken off line and heated.
The present invention process and apparatus has the additional
advantage of few moving parts and prolonged dehumidification
capability. Even though the accumulated reject water must
eventually be removed and energy must be expended, operation of the
device may be continued for prolonged periods without such water
removal. The reason this is permissible is that the water need not
be separated or re-evaporated at the same rate or at the same time
at which is it produced. If the water is directed outside, or where
a lower humidity waste stream is present, or preferably where a
source of waste heat is present (such as the condenser or
compressor of an air conditioning system), the water may gradually
evaporate with no additional work to be done by the system.
Yet another advantage of the present invention is that a method and
means for dehumidifying a gas stream is provided that can be used
with unusual contours, such as the ducts of air handling equipment,
the panels of cars, trucks, and ships as well as the walls of homes
and office buildings, the clothing of hikers and of physicians in
an operating room, and the bed linen of people who sleep in a humid
environment. A membrane can be molded to the contour of a boundary
of a volume to be dehumidified, with an osmotic solution contained
in a space adjacent to it. The present osmotic wall is also capable
of configurations within a rigid or flexible compartment, wherein
the wall is folded or otherwise arranged to provide an amount of
surface area that is significantly greater than, for example, a
planar wall extending across a compartment. As a result, the
overall flux through the osmotic wall is enhanced
significantly.
These and other objects, features, and advantages of the present
invention method and apparatus will become apparent in light of the
detailed description of the invention provided below and the
accompanying drawings. The methodology and apparatus described
below constitute a preferred embodiment of the underlying invention
and do not, therefore, constitute all aspects of the invention that
will or may become apparent by one of skill in the art after
consideration of the invention disclosed overall herein.
BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 is a schematic view of an air conditioning system
incorporating features of an embodiment of the present
invention.
FIG. 2 is a schematic view of an air conditioning system
incorporating features of an embodiment of the present
invention.
FIG. 3 is a schematic exploded view of an apparatus constructed in
accordance with the teachings of the present invention.
BEST MODE FOR CARRYING OUT THE INVENTION
FIGS. 1 and 2 schematically depict an air conditioning system 100
for conditioning the gas (e.g., air) within an enclosed space 102.
The system 100 includes dehumidification apparatus 104 (represented
by the components within the dotted line) and cooling apparatus
106. The dehumidification apparatus includes a dehumidifier 108 and
an evaporator 110. In this embodiment the evaporator includes an
auxiliary heater 112. The dehumidifier 108 is schematically
depicted as an enclosure 114 divided into an airflow compartment
116 and an osmotic fluid compartment 118. The compartment 118
contains an osmotic fluid, which is water with a solute dissolved
therein. The compartments 116, 118 are separated by a
semi-permeable osmotic wall 120 comprising an osmotic membrane 126
as will be described below.
Before providing a more detailed description of the properties and
characteristics of the semi-permeable osmotic wall and osmotic
fluid, and the interrelationships between them, it is helpful to
first have a general understanding of the overall operation of the
air conditioning system 100 within which they are incorporated.
The system 100 typically operates as follows: A fan 128 or other
suitable airflow generator pulls humid outside air represented by
the arrow 130, into an inlet duct 132 and blows it into and through
the airflow compartment 116. A valve 134 within the duct 132 may
direct all, some or none of that air around the dehumidifier
through a bypass duct 136, depending upon dehumidification
requirements. In this schematic representation, the valve 134 is
controlled based upon a signal from a sensor 135 which measures the
humidity of the air stream as it leaves the dehumidifier 108.
As the air passes through the compartment 116, water vapor in the
air condenses into liquid form within the pores of the osmotic
membrane 126. The water subsequently travels through the osmotic
membrane 126 into the osmotic fluid within the compartment 118. The
air, now lower in humidity, leaves the air flow compartment 116
through a duct 139 and is directed into the cooling apparatus 106,
which may be of any desired type. The cooling apparatus 106 cools
the air, if necessary or desired, and exhausts it via a duct 141
into the enclosed space 102 being conditioned, as depicted by the
arrow 137. Part of that cooled and dehumidified air may be
recirculated through the dehumidifier and cooling apparatus, such
as by returning a portion of it, via a duct 138, to the inlet duct
132, to be mixed with incoming outside air 130. A valve 140, or the
like, controls the amount of air recirculated, and may be
responsive to any number of parameters, such as the humidity and/or
temperature of either or both the outside air 130 and air within
the space 102. Although not shown in FIG. 1, the conditioned air
within the space 102 may also be recirculated through only the
cooling apparatus if dehumidification is not needed.
Further regarding operation of the system 100, the osmotic solution
leaves the compartment 118 via a duct 142, passes through the
evaporator 110, and is returned to the compartment 118 via a duct
144. As stated above, an evaporator 110 is an example of a means
for regenerating the osmotic fluid, and the present invention is
not limited to use with an evaporator 110. In the evaporator 110,
water within the osmotic fluid evaporates into the atmosphere at a
rate selected to maintain a desired water concentration within the
osmotic fluid in the compartment 118. As shown in FIG. 1, an
auxiliary heater 112 and a pump 146 within the duct 142 can be used
to affect the rate of evaporation. Their operation is controlled by
a signal from a sensor 148 that monitors the water concentration of
the osmotic fluid within the compartment 118. The auxiliary heat
needed to vaporize water in the evaporator 110 may be the waste
heat from the cooling apparatus 106, although this heat transfer is
not shown in FIG. 1.
An alternate method for maintaining the proper water concentration
in the osmotic fluid is to direct the osmotic fluid from the
compartment 118 into a large surface area overflow pan exposed to
the outside air. Fresh osmotic fluid with an appropriate water
concentration would be pumped into the compartment, as needed.
The semi-permeable osmotic wall 120 typically includes a
macroporous structure 149 that provides support to the osmotic
membrane 126. The macroporous structure 149 may comprise the same
material as the osmotic membrane 126, a different material, or some
combination thereof. The macroporous structure 149 may be disposed
on one or both sides of the osmotic membrane 126, or be integral
with the osmotic membrane 126. The macroporous structure 149 is
typically porous, having cells, pores, or the like that permits
water vapor to access the first side of the osmotic membrane 126
and/or osmotic fluid to access the second side of the osmotic
membrane 126, depending on the particular macroporous structure 149
used and its position relative to the osmotic membrane 126. FIG. 1
schematically shows the macroporous structure on the second side of
the osmotic membrane 126 for illustrative purposes.
The osmotic membrane 126 is a hydrophilic membrane having a
thickness typically in the range of between about five (5)
nanometers and one hundred (100) nanometers. The thinner the
membrane 126, the greater the water flux through the membrane 126
and into the osmotic fluid, since flux across the osmotic membrane
126 is inversely proportional to the thickness of the osmotic
membrane 126. Due to the large Gibbs Free Energy drive across the
osmotic membrane 126 exerted by the osmotic fluid, the pore
morphology of the osmotic membrane 126 allows water transfer
through the osmotic membrane 126 and into the osmotic fluid
compartment 118. That is because the water condensed in the pores
of the osmotic membrane 126 is water in the pure liquid state, and
the osmotic fluid is chosen to have a high concentration of solute.
The osmotic fluid exerts a type of osmotic "pressure" on the
condensed pure water. The magnitude of the osmotic pressure is
described by the Van't Hoff equation, while the osmotic pressure
gradient is in direct proportion to this magnitude and indirectly
proportional to the thickness of the osmotic membrane 126.
The preferred pore size for the osmotic membrane 126 will depend
upon the nature of the solute used in the osmotic fluid. As
mentioned above, the pores should not be so large that the hydrated
solute molecules can pass through them or enter and block the
pores. Pore diameters (i.e., separation distances between membrane
internal surfaces) on the order of about ten to twenty Angstroms
(10-20 .ANG.) would be acceptable for use with most osmotic fluids.
If a salt solution is used as the osmotic fluid, pore diameters of
between about five to ten Angstrom (5-10 .ANG.) are preferred. The
water flux across the osmotic membrane 126 is a function of the
membrane's permeability and the water concentration difference
across the osmotic membrane 126. Flux equals the product of
permeability, cross sectional area, and concentration difference
across the membrane 126. The permeability is inversely proportional
to the membrane 126 thickness.
The osmotic membrane 126 of the present invention is preferably
made from synthetic materials, including, but not limited to
synthetic polymers. Cellulose acetate and polyamide are examples of
acceptable synthetic polymers.
The dehumidifier 108, and more specifically the osmotic wall 120,
the airflow compartment 116, and the osmotic fluid compartment 118,
can assume a variety of different configurations. As shown
schematically in FIG. 1, the osmotic wall 120 may be disposed
within a box-like enclosure, separating the airflow compartment 116
and the osmotic fluid compartment 118. An alternative arrangement
schematically shown in FIG. 2 includes an array of osmotic walls
120 in the form of cylindrical tubes 121, wherein humid air 130
flows around and between the tubes 121 and osmotic fluid flows
through the tubes 121. Alternatively, the humid air may flow
through the tubes 121 and the osmotic fluid may be disposed outside
the tubes. Further alternative arrangements include flexible planar
osmotic walls, flexible osmotic walls folded into configurations
(e.g., non-random arrangements such as bellows and corrugations, or
a randomly folded bunch-type arrangement) that increase the surface
area of the osmotic wall extending between the airflow compartment
116 and the osmotic fluid compartment 118. The embodiment shown in
FIG. 1, for example, includes a planar osmotic wall having a
particular surface area. Additional osmotic wall surface area can
be provided in the embodiment of FIG. 1 by utilizing an osmotic
wall having a folded configuration rather than the planar
configuration shown. The increased surface area of the folded
configuration can permit a greater flow through the osmotic wall in
a given time period. As stated above, such flexible arrangements
may provide considerable utility in applications such as the
clothing of hikers and of physicians in an operating room, and the
bed linen of people who sleep in a humid environment. In still
further alternative arrangements, the osmotic wall 120 may be
molded to the contour of a boundary of a volume to be dehumidified,
with an osmotic fluid contained in a space adjacent to it.
An osmotic fluid having solute molecules that: 1) do not permeate
the osmotic membrane 126 in a dehydrated and/or a hydrated state;
2) have high solubility in water; and 3) do not degrade the osmotic
membrane 126, is an example of an acceptable osmotic fluid. Osmotic
fluids may be either ionic solutions or nonionic solutions.
Nonaqueous solutions may also be used. Examples of osmotic fluids
are lithium and magnesium salt solutions and phosphate salt
solutions, although other salts may be used. Examples of two
non-aqueous osmotic fluid solutions are glycerol and ethylene
glycol.
In the foregoing description, methods are described for maintaining
a high water concentration gradient across the osmotic membrane 126
that involve either evaporating excess water or to adding fresh
osmotic fluid to the osmotic fluid compartment 118. In a further
embodiment the osmotic fluid intentionally includes solute in
excess of the saturation limit. Thus, initially, solute crystals
will be present in the osmotic fluid. As water passes through the
osmotic membrane 126 and into the osmotic fluid, more solute will
dissolve, due to the presence of the additional water; and thus the
concentration of solute in solution will remain at the highest
level, i.e., saturation. Eventually, when all the undissolved
solute crystals dissolve and even more water enters the osmotic
fluid, the concentration of solute will gradually decrease and the
osmotic driving force will decrease, thereby reducing the amount of
water transported through the osmotic membrane 126. At this point,
the water must be removed and the osmotic solution
reconcentrated.
Examples of commercially available materials that can be used to
form the osmotic membrane 126 include "Polyamide RO AK Membrane"
and "Thin Film NF HL Membrane" both of which are manufactured by GE
Osmonics, and marketed by Sterlitech Corporation of Kent, Wash.,
U.S.A. for purification of brackish water by reverse osmosis. Other
commercially available materials that can be used to form the
osmotic membrane 126 include "X-Pack" and "Expedition" marketed by
Hydration Technologies, Inc. of Albany, Oreg., U.S.A. for
purification of water by forward osmosis.
Now referring to FIG. 3, to illustrate the dehumidification process
of the present invention, the internal volume 201 of a 100 ml
vessel 200 was filled with an osmotic solution comprising saturated
aqueous solution of lithium chloride. A number of different osmotic
walls 120, each approximately five centimeters (5 cm) in diameter,
were individually disposed between the top lip 203 of the vessel
and a compartment 204 open to the air on top of the top lip 203. A
tube 205 extending from the vessel 200 is used to measure the rate
of increase of water to the vessel 200. Humid air at relative
humidity between about 70% and 90% was blown over the top of the
vessel 200 and the change in liquid level in the tube 205 was
measured as a function of time for each wall 120. The osmotic fluid
was mixed with the incoming water using a magnetic mixer at the
bottom of the vessel 200. The results are indicated in the table
below:
TABLE-US-00001 OSMOTIC MEMBRANE WATER FLUX (liters/square
meter-hour) Polyamide RO AK 0.28 Thin Film NF HL 0.30 X-Pack 0.60
Expedition 0.80
From these experiments it was apparent that capillary condensation
occurs in regions of small pore size in each semi-permeable osmotic
wall 120, and that water traveled across the thickness of the
osmotic wall 120 likely via water bridges formed across the
thickness of the wall 120. Each osmotic wall 120 maintained a high
water concentration gradient by not allowing significant permeation
of the osmotic fluid in the direction of the humid air, thereby
resulting in water being driven through the osmotic wall 120 and
into the osmotic fluid. It is believed that the various osmotic
walls 120 exhibited different water fluxes because of their
different thicknesses and permeabilities. These characteristics
would influence the water bridges linking the liquid traveling
across the walls 120 and into the osmotic fluid, and therefore the
water flux rate.
Although the invention has been described and illustrated with
respect to the exemplary embodiments thereof, it should be
understood by those skilled in the art that the foregoing and
various other changes, omissions and additions may be made without
departing from the spirit and scope of the invention.
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